1. Field of the Invention
The present invention relates to medium access control (MAC) methods for wireless local area networks (WLANs). Specifically, the invention provides prioritization, time-bounded reservation, dynamic channel time allocation, admission control, and power management mechanisms to support applications with power saving and quality of service (QoS) requirements.
2. Description of Related Art
With the proliferation of portable devices and the advance of channel modulation techniques, there has been growing interest in providing QoS guarantees for multimedia applications over WLANs. A WLAN typically consists of an access point (AP) and a finite number of mobile stations (or called stations), such as wireless phones or laptops. On the other hand, mobile stations are often operated by batteries. It is well known that, due to technology limitations, the battery capacity will not be dramatically improved in the not-so-distant future. Hence it is important to design a MAC method with QoS and power saving support.
The international MAC standard for WLAN, IEEE 802.11, defines two modes of operation: distributed coordination function (DCF) and point coordination function (PCF). DCF employs carrier sense multiple access with collision avoidance (CSMA/CA) in the contention period (CP) for asynchronous data transfer. PCF employs the polling scheme during the contention-free period (CFP) to provide isochronous transmission services. In 802.11, PCF uses a point coordinator (PC), which should operate at the AP, to determine which station on the polling list currently has the right to transmit. When an AP/PC is operating in a WLAN, the two coordination functions alternate, with a CFP followed by a CP, which we will collectively refer to as a CFP repetition interval or a superframe. For a complete and detailed description, please refer to the IEEE 802.11 specification (IEEE Standard 802.11-1999, Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. IEEE, November 1999).
Although PCF can provide contention-free transmission, there are several problems with PCF that make it less attractive for QoS and power conservation. (i) Any station intending to receive contention-free service should first send the (re)association frame to the AP during the CP. Since DCF is governed by a contention-based DCF, the (re)association frames need to compete with all other stations in the same cell, resulting in an unbounded (re)association delay. Hence a real-time station with bad luck may never get on the polling list. (ii) Since DCF does not support traffic prioritization, so a low priority station may join the polling list earlier and faster than a high priority station. (iii) IEEE 802.11 does not allow a station to send frames directly to any other stations within the same infrastructure WLAN, and instead the AP should relay the frames always. In this way, the bandwidth is indeed consumed twice than directional communication between stations. (iv) During the CFP, the transmission time of a polled station is unpredictable and unrestrained. Any polled station is allowed to send a single frame that may be of an arbitrary length, up to the maximum of 2304 bytes (or 2312 bytes when the frame body is encrypted using WEP). This may adversely degrade and ruin the performance of the other stations on the polling list. (v) Since PCF has no admission control, the PC may admit a large number of real-time stations. Under the circumstances, several pollable stations may not receive a poll or the data from the PC during the entire CFP, thus incurring unnecessary awakeness and energy expense. (vi) When a pollable station wants to leave the polling list, it should reassociate with the AP via DCF. The station without additional buffered data but having no chance to get off the polling list will respond with a Null frame when polled by the AP. These Null frames are simply a wastage of bandwidth, thus causing the PCF performance down.
To accommodate additional QoS provision, IEEE 802.11e draft (IEEE 802.11e/D6.0, Draft Supplement to Part 11: Wireless Medium Access Control (MAC) and physical layer (PHY) specifications: Medium Access Control (MAC) Enhancements for Quality of Service (QoS), 2003) proposes a new coordination function, called HCF (hybrid coordination function). The HCF defines two channel access mechanisms: EDCA (enhanced distributed channel access) and HCCA (HCF controlled channel access). Especially, the EDCA introduces a new type of IFS (Inter Frame Space), named AIFS (Arbitration IFS), for different access categories. Since the AIFS values are longer than DIFS, the frame of a station using the existing DCF may take priority over that of a QoS-aware station using EDCA. In EDCA, the minimum and maximum values of the contention windows (CWs) of a high-priority station are smaller than those of a low-priority station, where CWmin denotes the smallest contention window and CWmax denotes the largest contention window. However, EDCA may suffer from the priority reversal problem; that is, a lower-priority station may take priority over a higher-priority station. Such a priority reversal phenomenon may discourage customers from buying high-priority WLAN services. We accordingly invent new MAC methods which can conquer the above-mentioned problems, coexist with DCF, and provide power saving and QoS guarantees to wireless multimedia applications as well.